The present invention relates in general to power supply regulation and, more particularly, to a switching regulator capable of starting the on-time of a power transistor once and then terminating the on-time of the power transistor anytime during a charging phase of a regulation cycle.
Most if not all electronic devices require a DC voltage of appropriate level for proper operation. The DC voltage is typically derived from an AC power source, e.g. by plugging a power supply into a wall socket. The AC voltage available at the wall socket is converted to a DC bulk voltage by a full-wave rectifier diode bridge. The DC bulk voltage is further converted to a regulated DC output voltage by a switching power supply.
The switching power supply uses a transformer, or an inductor depending on the configuration, as an energy transfer element. For example, a flyback-type power supply has a power transistor coupled to one side of the primary winding of a transformer. The power transistor turns on and off as determined by a switching regulator circuit to alternately store energy in the magnetic field of the transformer and transfer the stored energy to the secondary winding. The secondary winding of the transformer develops an output voltage across a shunt capacitor coupled in series with a rectifier across the secondary winding as a function of the energy transfer. The voltage across the capacitor provides the DC output voltage of the switching power supply.
The DC output voltage increases and decreases inversely with the power consumed by or delivered to the applied load. An increasing load decreases the DC output voltage and a decreasing load increases the DC output voltage. The DC output voltage, or a representation thereof, is fed back to the switching regulator circuit to allow the switching power supply to compensate for load variation. As the load increases, the DC output voltage decreases which causes the switching regulator to leave the power transistor on for a longer period of time to store more energy in the magnetic field. This additional energy is transferred to the secondary winding during the off time of the power transistor to supply the increased load and re-establish the DC output voltage. As the load decreases, the DC output voltage increases which causes the switching regulator to leave the power transistor on for a shorter period of time in order to store less energy in the magnetic field. The reduced energy transfer to the secondary winding during the off time of the power transistor causes the power supply to adjust to the decreased load and reduces the DC output voltage back to its steady-state value.
Some prior art switching regulator circuits are configured as gated oscillators where the power transistor receives a fixed frequency control signal which is enabled or disabled in response to the feedback signal. Each regulation cycle of the gated oscillator is initiated by a clock pulse signal setting a latch. The clock pulse signal initiates the regulation cycle by enabling the gate drive signal, and correspondingly the on-time of the power transistor, if the feedback signal is not asserted. The gate drive signal is terminated either by a current limit sense signal detecting peak current in the power transistor, or by a falling edge of an oscillator signal resetting the latch. If the feedback signal is asserted during the time when a regulation cycle should start, i.e. at the rising edge of clock pulse, then the gate drive signal is not enabled as the clock pulse signal to the latch is blocked by the feedback signal. The power transistor is not turned on and no energy is stored in the magnetic field of the transformer during that regulation cycle.
The switching regulator can react to changes in output loading only at the start of the regulation cycle. For example, if the feedback signal is asserted at the beginning of the regulation cycle, indicating that the DC output voltage is already above the regulation threshold, then the power transistor is not turned on until at least the next regulation cycle. Therefore, if feedback information indicating an increasing load is delivered immediately following the clock pulse, the power transistor remains non-conductive during the regulation cycle even though the load is demanding more power.
The gate drive signal and on-time of the power transistor during the regulation cycle is typically terminated either by peak current limit or by maximum duty cycle. For example, if the feedback signal is not asserted during the regulation cycle, indicating that the DC output voltage is below the regulation threshold, then the power transistor is turned on at the beginning of the regulation cycle. The power transistor is not turned off until terminated by peak current limit or the end of the maximum duty cycle. Therefore, if feedback information indicates that a load is suddenly removed and the output voltage is increasing, the power transistor continues to store energy in the magnetic field until terminated by peak current limit or maximum duty cycle, even though the load requires no additional energy.
The narrow window of opportunity to initiate a regulation cycle by enabling the gate drive signal and the inability to terminate the gate drive signal as indicated by the feedback signal causes poor transient response to variations in output loading during the regulation cycle.
Hence, a need exists for a switching regulator circuit which can respond to variations in output loading anytime during the regulation cycle.